The present invention relates generally to systems and processes for heat treating carbonaceous materials to recover hydrocarbon products. More specifically, the present invention relates to a process and system for removing moisture from carbonaceous materials prior to heat treatment by pyrolysis or otherwise.
Pyrolysis of carbonaceous materials such as oil shale and coal, is a process commonly in use for recovering valuable hydrocarbon products. Pyrolysis basically involves heating the carbonaceous material to elevated temperatures normally above 800.degree. F. in a non-oxidizing atmosphere. The carbonaceous material is devolatilized or degraded at this temperature into various hydrocarbon products which are recovered. It has been found advantageous in pyrolysis systems to initially preheat the carbonaceous material to temperatures on the order of 500.degree.-600.degree. F. Many different types of preheat systems are presently being used ranging from moving bed or fluidized bed preheaters to staged preheat systems in which the carbonaceous material is entrained in a series of gas streams having gradually increasing temperatures.
Raw carbonaceous materials such as coal and oil shale generally include a certain amount of moisture. The amount of moisture generally ranges between 1 and 35 weight percent. Although it is possible to directly pyrolyze moist carbonaceous materials, it has been found desirable to initially remove moisture present in the carbonaceous material prior to pyrolyzing.
In order to provide a suitable drying process or system, the process must be capable of conveniently and efficiently drying large amounts of carbonaceous material in a minimum amount of time with little or no devolatilization or degradation of the carbonaceous material. Any number of well-known preheat type processes may be utilized in initially drying moist carbonaceous material. For example, fluidized beds, or rotating drums may be operated at mildly elevated temperatures to provide the desirable removal of moisture from the particulate material. In addition, the drying processes may include systems where the carbonaceous material is entrained in gas streams at elevated temperature.
In the gas stream entrainment type dryers, the total solids blend of particulate material, including a distribution of smaller and larger sizes, is exposed to contact with hot gases in a lift pipe or the like. Moisture and/or any solvent present evaporates from the material into the gas phase. Transfer of heat from the hot gases to the solids is a function of fluid flow, gas thermal properties, solids thermal properties, solid surface characteristics, temperature differences and gas-solid residence time. In the entrained flow-type system, the smaller particles achieve higher velocities and, as a result, the rate of heat transfer is greatest for these particles in any given section of the dryer.
Normally, the temperature of the particulate solids in the gas stream increases to the point at which the moisture begins to evolve. It should be pointed out that in this specification, the term moisture is intended to include not only aqueous moisture but solvent moisture of various types present in raw coal or oil shale which have vaporization temperatures below initial devolitilization temperatures. The evolution or vaporization of moisture continues at a relatively constant temperature until all of the moisture has evaporated from a given particle. The moisture may be uniformly distributed throughout the particles or may only be surface related. In any event, because of the rate of heat transfer to the smaller particles and their smaller total mass, they reach total dryness much sooner that the larger particles and rapidly approach contacting gas temperatures. In order to dry carbonaceous materials in a minimum amount of time, it is desirable to maintain contacting gas temperatures as high as possible. Accordingly, gas stream temperatures are maintained well above the devolatilization or degradation temperature of the carbonaceous material. As a result, in entrained flow-type dryers, the smaller particles are generally overheated and partially degraded or devolatilized resulting in loss of hydrocarbon product.
Although entrained flow-type drying systems are well-suited and particularly advantageous for removing moisture from particulate solids in a convenient and quick manner, it would be desirable to provide an improved entrained flow dryer in which hydrocarbon product losses due to overheating of smaller particles is reduced.